Federal authorities said yesterday the testimony of an ex-cop will prove that former Officer Charles Schwarz was part of a police conspiracy in the Abner Louima case — but the witness himself branded the idea “insane.”

Anthony Abbate, a former Patrolmen’s Benevolent Association delegate, has been called to testify at the upcoming conspiracy trial of Schwarz and two other cops, according to a prosecution letter filed in Brooklyn federal court.

“Abbate is a hard-line, old-school PBA guy with a lot of juice,” a source told The Post. “He was in on the ground floor of the cover-up in the Louima case.”

Abbate insisted his testimony will prove nothing about Schwarz. The prosecution claim is “absurd, absolutely insane,” he said. “There was no conspiracy whatsoever. It’s a horrible thing to accuse somebody of.”

The letter suggests Abbate will testify about conversations he had with Schwarz when 70th Precinct cops found out they were under investigation.

Schwarz’s lawyer, Ronald Fischetti, has vowed to use the trial next month to show his client was wrongly convicted last spring of holding Louima down as Officer Justin Volpe sodomized him with a stick in the station-house bathroom. Schwarz, who faces life in prison, has not yet been sentenced.

Volpe, who was given 30 years, insists another cop, Thomas Wiese — not Schwarz — was in the bathroom.

Wiese and Officer Thomas Bruder are the cops charged with conspiring with Schwarz.

Prosecutors say Abbate’s testimony will show Schwarz was part of a police plot to cover up his role in the attack.

Abbate said, “We didn’t connive to do anything. I lent a sympathetic ear, like any friend or family member would.”

Abbate was fired after being convicted at a departmental trial of cursing another officer. Schwarz was a defense witness at that proceeding.

Fischetti called the prosecution letter “absolute nonsense,” and said it showed prosecutors had little evidence against Schwarz.